Field of the Invention
The present invention relates to an electrode terminal which is ultrasonically bondable to a bonded member, a semiconductor device including the foregoing electrode terminal, and a power conversion apparatus including the foregoing electrode terminal.
Description of the Background Art
A semiconductor module (semiconductor device) used for power control, which is equipped with a power semiconductor element within a package, is called a power module. As a power semiconductor element, for example, a switching device such as an insulated gate bipolar transistor (IGBT) and a metal oxide semiconductor field effect transistor (MOSFET), a freewheel diode (FWDi), or the like is employed. In such a power module as described above, conventionally, for leading out of an electrode from a power semiconductor chip, a bonding method such as wire bonding and direct lead bonding is used.
In wire bonding, it is general that a wire having a diameter in a range of several tens to several hundreds μm is bonded, and a plurality of wires are bonded in order to ensure an allowable amount of a current required for energization.
However, because of a constraint to a physical dimension of a bonding apparatus, there is a limit to bonding of a plurality of wires at a high density, so that the desired number of wires cannot be bonded in some instances. As a result of this, a density of a current flowing in each wire is increased, which then increases heat generation in a wire during energization. This causes a problem of reduction in a life of bonding between a power semiconductor chip and a bonding surface, and another problem of a need to enhance heat resistance of members provided around a wire. Also, an impedance component in wiring increases as the number of wires is reduced, so that desired electrical characteristics cannot be attained in some instances. For example, in a case where impedance of electrode wiring is large, a surge voltage which is caused at a time of a high-speed switching operation of a power semiconductor element is increased, so that usable regions of a voltage and a current in a power semiconductor element are significantly limited in some instances.
As a bonding method which is different from wire bonding described above, there is cited direct lead bonding in which an electrode terminal is soldered directly onto a power semiconductor chip (Japanese Patent Application Laid-Open No. 2009-200088, for example). According to this bonding method, an electrode which has a larger cross-sectional area and a larger bonding area than a wire for wire bonding is bonded, so that effects of reduction in heat generation resulted from reduction in current density, an improved life of a bonding part, ensuring of a margin of heat resistance of a member provided around an electrode and a chip, and reduction in impedance can be attained.
However, in order to achieve solder bonding in direct lead bonding described above, it is necessary to change a surface of a chip to a metal material such as “gold” which is suitable for soldering. To make such a change, it is conceived to form a bonding layer including “gold” on a bonding surface of a power semiconductor chip. However, formation of a bonding layer creates a need for an additional process such as sputtering, to cause a problem of complicating manufacturing processes.